Marketable Securities - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Investments, Debt and Equity Securities [Abstract]
Fixed income securities	$ 389	$ 5,300
Purchases of marketable securities	741	25,236
Proceeds from sales of marketable securities	5,676	20,913
Realized gains on sale of marketable securities	12	189
Unrealized holding gain arising during the period	$ 6	$ 0
Marketable securities, effective maturity period (in years)	8 months 27 days
Marketable securities, average effective duration (in years)	1 month 13 days